1. Field of the Invention
The present invention relates to a process for obtaining three-dimensional images of an object, as well as the application of this process to the tomography of an organ. It more particularly applies to the reconstruction of three-dimensional images to an organ in the human body, on the basis of the detection of gamma radiation from a tracer injected into the organ. It can also be used in the reconstruction of three-dimensional images from the detection of radiation from an object or an organ.
2. Description of the Prior Art
It is known to restore the radioactive distribution of a three-dimensional object, in accordance with a succession of planes having random orientations relative to said object. The acquisition of the initial data is based on the measurement, with the aid of a scintillation camera or scintiscanner, which is sensitive e.g. to gamma radiation, of photons emitted by a radioactive tracer injected into the organ or object, which it is desired to examine. This measurement is carried out during a 360.degree. rotation of the camera about a major axis of the organ or object, which it is desired to examine. For each of the camera rotation angles about the object or organ, a so-called scintigraphic image thereof is obtained. In general, the camera used is a scintillation camera, whose collimator is constituted by a lead plate having a large number of cylindrical or hexagonal holes with parallel axes, giving access to a radiation detector, a group of collimator holes facing an elementary detection zone supplying a detection signal in the presence of radiation. This collimator makes it possible to eliminate photons, which are emitted in directions not perpendicular to the plane of the detector. The scintigraphic image obtained results from the sum of the photons from the elementary cells of the organ or object, which emit radiation in directions perpendicular to the plane of the camera and which are therefore parallel to the collimator channels. The elementary cells are aligned in these directions. Thus, the obtaining of each scintigraphic image for each position of the camera, results from radiation emitted by elementary emitting cells located on straight lines perpendicular to the plane of the camera. For each of the angular positions of the camera about the organ to be examined, the electrical signals from the detector are proportional to the number of photons received in directions perpendicular to the camera plane. As a function of the different elementary detection zones, these electrical signals are digitized and recorded in an e.g. magnetic memory for use in the reconstruction of images of the organ to be examined, for the different angular positions of the plane of the detectors relative to the organ or object to be examined.
The digitized signals recorded in the memory are sorted as a function of each angular position of the plane of the detectors about the organ, for a rotation of 360.degree. of the said plane. The digital values are processed by a special algorithm, so as to restore the radioactive distribution of the tracer contained in the organ or object, in accordance with a so-called "reconstruction" method. In order to obtain sectional images for different sectional planes perpendicular to the detector plane, every effort is normally made to perform a minimum number of processing operations on the signals from the detector, using a minicomputer, without precisely taking into account the physical limitations limited with the radiation detection system, i.e. the limitations associated with the camera. In addition, no account is taken of the effect of autoabsorption of the radiation within the organ or object to be examined. All these approximations are prejudicial to the quality of the images obtained. For the energy of the gamma radiation currently used in nuclear medicine, this autoabsorption phenomenon is the most serious cause of the deterioration of quantitative data having to be processed in order to obtain a correct image of the body to be examined.